1. Field of the Invention
The present invention relates to a recording medium transport device comprising a transmission mechanism such as a gear mechanism, a belt transmission mechanism, a chain transmission mechanism or the like between a drive source and a registration roller.
2. Description of the Related Art
So far, a sheet transport devices such as a document transport mechanism and a print paper transport mechanism of an image forming apparatus has been widely known. The image forming apparatus, for example an electrophotographic type image forming apparatus, comprises an image bearing body, an image writing part, a developing device, a sheet feeding cassette, a transferring part, a fixing device or the like. The image writing part forms an electrostatic latent image onto the bearing body that is uniformly electrically charged, and the developing part develops the latent image to a toner image. One transfer sheet as a print paper is transported from the sheet feeding cassette, and the toner image formed on the image bearing body is transferred to the transfer sheet at the transferring part. Then, the transfer sheet is transported to the fixing device, and the toner image is fixed to the transfer sheet. The transport mechanism comprises registration rollers (timing rollers) for guiding the transfer sheet to the transferring part at a predetermined timing. The transfer sheet contacts a nip portion of the registration rollers for skew correction, enabling to perform image formation in the correct position and direction.
Drive force is transmitted from the drive source such as a drive motor or the like to one of the registration rollers through a plurality of gears, and there is a clearance (looseness) so called backlash exists at a meshing portion of gears or an engaging portion of a gear and a shaft of the registration roller.
After transferring the transfer sheet to the image forming part, the registration rollers are stopped for making the tip portion of the next transfer sheet contact the registration rollers, however, the registration rollers rotate by the amount of the backlash with the moment of inertia of the registration rollers themselves at this time. In the case that the registration rollers are stopped at a position which is ahead by the amount of the backlash, when the drive source starts to drive the registration roller again, the gears or the like spin out by the amount which the registration rollers lead before the drive force is transmitted to the registration rollers. Thus, there is a time difference generated between the time when the drive source starts to drive and the time the registration rollers actually starts to drive.
If the amount which the registration rollers lead before the drive force is transmitted to the registration rollers is constant, there would be no problem. However, practically, the registration rollers stop at a different position every time, so that the amount which the registration rollers lead is not always constant. As described above, when the amount which the registration rollers lead is not constant, it would cause variation of the timing to carry a transfer sheet, therefore, a toner image may be transferred to the transfer sheet at a position where the toner image is misaligned in the transport direction.
The variation of the timing to transport a transfer sheet would be caused when handling jamming which happened near the registration rollers as well as when driving and stopping the registration rollers as described above. When jamming happens, the jammed transfer sheet needs to be removed by opening a door of the image forming apparatus. At this time, a user or a service man may rotate the registration rollers in a direction opposite to the transport direction of the transfer sheet, which would cause a gap in the timing to transport a transfer sheet.
There is known an image forming apparatus disclosed in Japanese Unexamined Patent Publication Tokkai-2002-265097A which can prevent displacement of a toner image transferred onto the transfer sheet. In such image forming apparatus, a transfer sheet contacts a specific contact surface which is provided closer to an image forming part than the registration roller. The contact surface can be evacuated from a transport path by a solenoid.
However, the technique in the above described Tokkai-2002-265097A uses the specific contact surface for contacting a transfer sheet, and the solenoid for disposing or evacuating the contact surface at or from the transport path. This would result in increasing the number of components and making the control complicated, thereby raising the cost for manufacturing an image forming apparatus.
A gap in timing to transport a sheet is a problem even in an image forming apparatus which does not require transferring such as an ink jet type image forming apparatus, or in a document transport apparatus such as an ADF, so that it is desired to transport sheets at correct timing without raising the cost for manufacturing an apparatus.
As a transmission mechanism between the drive source and the registration roller, a belt transmission mechanism, a chain transmission mechanism or the like has been known other than a gear, however, these mechanisms would also cause a phenomenon similar to the backlash. That is, there is looseness in a belt which is wound on a pair of pulleys or in a chain wound on a pair of sprockets, and that looseness would cause a phenomenon similar to the backlash. Accordingly, even when the belt transmission mechanism or the chain transmission mechanism is used as a transmission mechanism between the drive source and the registration roller, an idea for realizing to transport sheets at a correct timing at low cost is desired.